[ginger]

Andy,
An alternating magnetic field will induce currents in anything able to conduct those currents, that is anything conductive.

If you are trying to shield stuff from magnetic fields then you need to use a material with magnetic properties. The "traditional" material for magnetic shielding is "mu-metal". Do a web search on "mu-metal" or "magnetic shielding" and you will probably find about 1000% more than I know about such stuff.

When I needed a magnetic shield for the photo-multiplier tube of a Laser Receiver I just specified that it had to be a cylinder of a diameter suitable for slipping over the tube and of a particular length AND made from mu-metal. I did'nt bother to specify it any more exactly than that.

Cheers,
Ginger

[Tinker]

If you are trying to shield stuff from magnetic fields then you need to use a material with magnetic properties. The "traditional" material for magnetic shielding is "mu-metal". Do a web search on "mu-metal" or "magnetic shielding" and you will probably find about 1000% more than I know about such stuff.

I've used a mu-metal shield once before too. It has a coercivity and permeability something like 2 orders of magnitude better than tin plated steel. You can also find it under the trade name "Permalloy," ie high permeability alloy.

One bugger is that if you bend it it loses much of its magic properties, it has to be annealed in the correct shape.  Although there is a foil you can get which will tolerate a fair bit of flexing.

If you want to get really sticky see if you can find a substance called "Supermalloy."

T.

[andyr]

Andy,
An alternating magnetic field will induce currents in anything able to conduct those currents, that is anything conductive. ...

Hi, Ginger,

Thank you for your explanation of Faraday-cage mesh mathematics.  Glad to know that the commonly available mesh types should be absolutely fine!  (So I don't have to take a crowbar to my microwave oven!)  

However, you need to be a bit gentle with your fellow homo sapiens who don't have a background of 30 years of EE theory ... a bit more detail, please!!  

So there is no difference in terms of induced eddy currents between mild steel, stainless steel, aluminum (as our Yankee friends call it!) or copper?  

Is there any other intrinsic property of a metal that would result in it being a more "benign" (read: less sonics-damaging!) case material than mild steel?

I ask because Naim use aluminium and other manufacturers use stainless steel ... or are these just cosmetic decisions?

Regards,

Andy

[Seano]

Hey Andy (and anyone else interested in such esoterica)

Have a womble around this website for a bit of mindbending on the issue of interference shielding http://www.rfi-ind.com.au and some fairly fancy sounding stuff to reduce RFI/EMI and EMC and other mad things that I didn't know were such a big deal.

[andyr]

Hey Andy (and anyone else interested in such esoterica)

Have a womble around this website for a bit of mindbending on the issue of interference shielding http://www.rfi-ind.com.au and some fairly fancy sounding stuff to reduce RFI/EMI and EMC and other mad things that I didn't know were such a big deal.

Hi, Seano,

Mind-bending indeed ... I needed half a dozen G&Ts after wombling around!  

Regards,

Andy

[soitstarts]

Hmmm, don't realy use the scanner much anymore. I wonder???

[andyr]

Hmmm, don't really use the scanner much anymore. I wonder??? ...

C'mon, mate,

Don't you have access to the machine shop at the mine?  Only take you a month's worth of evenings to make your own mesh like in the picture, drilling thousands of holes in a 500mm square piece of steel!

Just the thing on those cool winter evenings ... keeps you off the grog, too!  

Regards,

Andy

[soitstarts]

Don't you have access to the machine shop at the mine?  
Just the thing on those cool winter evenings ... keeps you off the grog, too!  


Andy

Machine shop??? They sold all thier stuff to a contractor for $1.00 years ago. Only have a guilotine and bender now... (Think they regret it now...)

Cool winter evenings???? At the moment, on night, shift its 38degs. Day shift its 45 degs in the shade. Iron men of the Pilbara, thats us. More like hot and bothered men of the Pilbara..

Dont need to do much drilling anyway, We've got hyd. filters that have have 3mm holes on the metal reinforcement. Just about the right size to fit a torroid in too..  

Got the missus to keep me off the grog, too...

[jules]

Is there a slight sense of "I don't believe in this but I am prepared to try it" in this discussion    I don't say this critically because I had it in mind when I designed my own amp in terms of using wood and also making it reasonably thick.

The key issue here doesn't seem to be a question of shielding so much as "un-schielding" if I can put it that way.

If you take away a metal case, you take away shielding against a number of known problems ... EMI and RFI for example, so yes, there is going to be a problem with these if they are present but that is not always going to be the case.  

Perhaps metal cases could actually compund a problem. If they are a shield, they are also a trap and maybe even an amplifier for the magnetic and other internally generated

If taking away a metal case does give more clarity then there are two possibilities :-

   * the case is a trap or even an amplifier for internally generated problems

or

   * the case transmits externally generated interference in such a way as to interfere with the amp. [Metal is not a shield against everything just as glass isn't either. Clearly metal is a fantastic conductor of a wide range of  audible sound].

I doubt there's anything particularly mystical about all this but perhaps in all this concern over EMI and other probs that can be shielded out by various forms of metal we are dismissing a problem that is actually caused by metal.

jules

PS Kyrill has made a number of posts on C37 theory which might have relevance here. At the same time, I'm not sure that putting a foil layer [eg gold] inside a case might not simply make the case "metal" in some sense.

[kyrill]

Hi

rule 1) Ben is right. You must first block your amp's innards from outside pollution as much as you can by following the cage of faraday principle. It is impossible to avoid this.
just as you should protect the pws current from the pollution out of the wall socket. This is why shielded tubes sound more "clean" than unshielded tubes unless the negative effects of rule 2) contaminates the advantage of shielding

rule 2) Then the case/shield of the amp does not stand still. It vibrates by itself mostly by the soundwaves coming out of the speaker. pArticular near its own resonance frequency.. The electronics will be affected two ways"
a) the magnetic vibrating metal case in the middle of the e.m.f of the electronics will induce a complex wave of  miniscule voltages and current  all over the pcb. Earthing the case will not help.
Non magnetic cases will do this not if i understand this right or to a lesser degree
b) The resonance freqs of the case, depending on its material and structural properties, will reach the pcb in a mechanical way. because normally the pcb is rigidly coupled to its base and therefore the the whole cage. The pcb picks up easily the resonance freq. of the case and generates with its own vibration in the same e.m.f. voltages and curents upon itself
C37 theory takes over now. Some resonances (wood) are musically pleasant albeit an unwanted addition to the pure signal amplification and some resonances are (very) unpleasant (steel/aluminium)
"..At the same time, I'm not sure that putting a foil layer [eg gold] inside a case might not simply make the case "metal" in some sense."'
The glued foil will follow the resonance freqs. of the wood.              

This is all theory and will hardly be noticed if not the whole amplification chain including the input does not  have a very very low "distortion" floor. This is normally the case with highly transparent systems. The AKSA and GK-1 are part of that class

[econ]

is lead sheeting  useful for shielding ?
It can be bought in rolls for a few bucks in any hardware store

[andyr]

is lead sheeting  useful for shielding ?
It can be bought in rolls for a few bucks in any hardware store

No, econ,

I don't think lead is much use for the kind of shielding which AKSAs need.  To stop the (internal) power transformer magnetic fields from getting to the signal, mild steel is required (because steel is magnetic); to stop external EMI from getting to the signal, an earthed metal case ("Faraday shield") is required.

But then I am an amateur ... maybe a "pro" will give U a more soundly-based answer.

Regards,

Andy

[Greg Erskine]

I think lead could be used for:

1. a Faraday cage
2. damping/deadening material
3. protection against x-rays
4. protection against kryptonite
5. giving children brain damage.


 

[Grumpy_Git]

I've personally taken to asbestos, it's a beautiful material, very rigid and the lead paint i use on it gives a piano gloss finish.  

Another plus is the heat resistance if i ever decide to embrace tubes and get a GK1.  

gotta go, supposed to be working.

Nick.

[jules]

The results of a little research on magnetic shields:-

DISTANCE AND MAGNETIC FIELD STRENGTH, this is the easy bit: The strength of a magnetic field generated in a wire of a particular diam eg 1mm, falls away proportionally so that [with the 1mm wire] at 5mm it has 20% of it's strength at the surface of the wire, at 10mm it's 10% and at 100mm it's 1%.

MAGNETIC SHIELDING [which can only be achieved by substances that can be magnetized] does not remove magnetic fields, it redirects them. The best shape for shielding is closed ie closed box or cylinder with top and bottom. Shielding has to offer a complete path for field lines so that they don't exit at the wrong place and cause interference.

Seems to me shield design could be trickier than it looks. At the worst it would be possible to create a secondary magnetic field outside a transformer that is worse than using no shield!

jules[/u]

[Seano]

I've personally taken to asbestos, it's a beautiful material, very rigid and the lead paint i use on it gives a piano gloss finish.  

Another plus is the heat resistance if i ever decide to embrace tubes and get a GK1.  

I'm considering using a carefully prepared 44 gallon drum that used to contain jet fuel - nice and easy to clean. Chemically inert plastic lining, heavy steel exterior, it'll look quite shmick when carbonate blasted then hit with seven coats of pearlescent candy cherry 2 pak enamel then three coats of clear. No silly magnetic fields will get in or out of that one.  Cheap too!

[jules]

something missing there Seano ... ah, got it ... should be finished off with the trademark jarrah panel, in this case probably a large cross section of a tree  

Given the stuff I wrote above about magnetic shielding, consider this:-

If you have a toroid in a confined space and you make a flat metal screen screen that almost touches the toroid at its closest point and then goes on so that one end of the screen finishes up in the vicinity of your pcb what you might have created is an electromagnet picking up its field from where it is close to the toroid and directing it neatly into the pcb. This would also depend on the metal used in the shield but if it wasn't something very specific like Mu metal it could be suspect. "Steel" can range from being totally suitable to being totally unsuitable depemding on its make-up.

jules

[Seano]

something missing there Seano ... ah, got it ... should be finished off with the trademark jarrah panel, in this case probably a large cross section of a tree  

Could always forgo the candy cherry paint and stick some of that vinyl jarrah veneer they use on cheap speakers on to it instead

In truth though I'm thinking of going the Halcro route to an AKSA integrated amp. A stand up piece of timber furniture with a CD player sitting on top.  Inside the cupboard (so to speak) and largely out of view is the DAKSA, GK-1M and four channels worth of 100N+ amp. PS's for the lot with be in a SS box in the bottom of the cupboard. Plenty of room to wire up bits , remove/replace and generally tinker............dreamin'

[kyrill]

The AKSA's are worth the best approach

SO I use the transformators and their "difficult"magnetic fields in a seperate EMI and RFI shielded box. The distance now between the box and the AKSA pcb's is appr. 1 mtr (2 yrds)

[Greg Erskine]

In truth though I'm thinking of going the Halcro route to an AKSA integrated amp. A stand up piece of timber furniture with a CD player sitting on top.  Inside the cupboard (so to speak) and largely out of view is the DAKSA, GK-1M and four channels worth of 100N+ amp. PS's for the lot with be in a SS box in the bottom of the cupboard. Plenty of room to wire up bits , remove/replace and generally tinker............dreamin'

I've been thinking along similar lines myself but with slide-in shelfs or drawers and no back. Each piece of equipment built directly onto an individual shelf. I would probably have it on wheels so I can turn it run to make the connections. I was hoping to find a nice set of drawers or a cupboard that could be easily modified.